In recent years, due to demand for energy saving and shorter warm-up time (i.e., the amount of time between when the image forming apparatus is turned on and when the fixing device can start the fixing operation) in a fixing device, image forming apparatuses using a belt-type fixing method in which smaller heat capacities can be set have attracted attention. Also in recent years, image forming apparatuses using an electromagnetic induction heating method which provides quick heating and high-efficiency heating have attracted attention. In the context of saving energy required for fixing color images, many products that combine the belt-type fixing method with the electromagnetic induction heating method have been commercially available. When the belt-type fixing method and the electromagnetic induction heating method are combined, a device that generates magnetic flux for electromagnetic induction is often provided outside a belt (so-called external induction heating (IH)). The use of this arrangement is advantageous in that a coil can be easily laid out and cooled and the belt can be directly heated.
In the electromagnetic induction heating method described above, various techniques have been developed to prevent overheating in a non-sheet-passing region in accordance with the width of a sheet that passes through the fixing device (sheet passing width). In particular, a size switching technique in external IH is known. In this technique, a ferrite center core that constitutes a part of a magnetic path is provided around a coil. As the center core rotates, a selection is made as to whether the belt is to be subjected to induction heating caused by magnetic flux generated by the coil, or induction heating is to be suppressed by cutting off the magnetic flux. With this technique, the amount of heat generation in the belt in the non-sheet-passing region can be set to a value different from that in the sheet passing region.
To create a magnetic path in a region for a maximum sheet size, the center core is formed as a single long narrow body that extends along the rotational axis thereof. In this case, unless the center core is manufactured with high accuracy, rotational vibrations of the center core may become large and variations in distance between the center core and the belt may be caused, and may thereby result in uneven heat generation in the belt in the direction of the rotational axis of the belt. If the center core is manufactured by cutting, it may be difficult to reduce manufacturing costs. If the center core is molded with a mold, high dimensional accuracy may not be achievable. Therefore, it is possible to divide the center core into a plurality of core bodies, which are then arranged on a shaft.
However, above-mentioned conventional technique still needs to be improved in terms of assembly of the center core. This is because when the center core is divided into a plurality of core bodies, there are manufacturing dimensional variations among the core bodies. Specifically, when the core bodies are manufactured by pressing and sintered powder, the shrinkage ratio (in the radial and axial directions) varies from one core body to another.
More specifically, when these separate core bodies are simply arranged on the shaft, core bodies located at both ends of the shaft will easily protrude from the shaft. In particular, when the core body is longer in the axial direction than in the radial direction, the influence of shrinkage ratio in the axial direction is more significant.